This invention relates to the preparation of novel derivatives of cellulose having improved retention characteristics not only of water but also of physiological solutions, for example, urine or blood. In one particular embodiment, it relates to the production of highly absorptive forms of cellulose derivatives obtained by graft polymerization of vinyl or acrylic monomers or derivatives thereof onto cellulosic fibres, and to the highly absorptive cellulosic derivatives so formed.
In recent years there has been a rapidly growing market for disposable absorbent products such as, for example, diapers, sanitary napkins and surgical dressings. This has focussed attention on the properties and preparation of the fibre materials used to make these products. Up to the present time, conventional wood pulps have provided a large share of the fibre required as the absorptive component. However, there have been increasing efforts recently to obtain better absorptive properties than those offered by conventional wood fibres, with a view to achieving both an improved end-product and economies in manufacture.
Polymer-modified cellulosic materials containing carboxylic acid groups or hydrolyzable functional groups are now known in the art. Examples of patented procedures include those disclosed in: Canadian Pat. Nos. 756,045 and 756,046 each issued Apr. 4, 1967 to J. W. Adams and H. W. Hoftiezer; Canadian Pat. No. 793,369 issued Aug. 27, 1968 to Tee-Pak, Inc.; U.S. Pat. No. 3,553,306 issued Jan. 5, 1971 to J. A. Church; and U.S. Pat. No. 3,194,727 issued July 15, 1965 to Tee-Pak, Inc. All the above-noted procedures are general to the in situ polymerization of an olefinically unsaturated monomer containing carboxylic groups (e.g. acrylic acid or salt) or hydrolyzable functional group, e.g. ethyl acrylate, butyl acrylate or acrylonitrile. The water absorptive properties of wood pulp modified by graft polymerization of polyacrylonitrile (known hereinafter as PAN) followed by alkaline hydrolysis of the PAN-grafted fibres and subsequent washing to pH 8 to 9 has been described in J. Appl. Pol. Sci., Vol. 17, No. 10, October 1973, p. 3143. As described therein, the product comprises modified fibres which are very swollen in water and hold the water tenaciously. Water retention values after centrifugation under 900 G. for 30 minutes of up to 30- 40 grams water per gram fibre are obtained, the amounts being dependent on graft level.
High water and physiological solution absorbency and strong binding power are of great interest in absorbent sanitary products, but the utilization of the above-described fibres in such products has been very restricted due to the difficulty and high cost associated with the removal of the substantial amounts of water retained by the hydrolyzed fibres. Conventional thermal drying procedures were found to be slow, even at elevated temperatures because the water is tenaciously held and its diffusion out of the fibres is slow. Furthermore, drying at elevated temperatures proved costly and imparted undesirable brittle properties to the fibres. Thus, if a sheet or pad of these fibres were dried, very strong interfibre bonding developed and the pad or sheet was brittle and could not be easily disintegrated into a fluff, which is the form under which fibres are generally used in absorbent products.
One procedure suggested to prevent interfibre bonding and to obtain a dry, fluffy fibre is the freeze-drying method. However, this procedure suffers the technical and commercial disadvantage that it is slow and expensive.
Another procedure suggested to prevent interfibre bonding and to accelerate the drying is the solvent-exchange drying of the hydrolyzed fibres. However, since the cost of solvent-exchange drying is a function of the quantity of solvent utilized and thus of the quantity of water to be removed, such solvent-exchange drying procedure also suffers the technical and commercial disadvantage that it is very expensive. The quantity of water to be removed can be reduced, if the solvent-exchange drying operation is carried out on hydrolyzed, filtered but unwashed fibres, the liquid retained being 8 to 10 g/g. The solvent-exchange drying may be carried out with a solvent miscible with water, non-swelling for the polymer and in which the alkali used for hydrolysis is soluble, so that the excess alkali is removed at the same time. Methanol is a good solvent for such purposes. Such solvent-exchange dried fibres are fluffy and after being placed in water and centrifuged to 900 G will have a satisfactory water retention value and thus can be used as water absorbent material. However, when they are placed in aqueous 1% NaCl, a solution used to simulate physiological solutions such as, for example, urine or blood, such fibres will not reswell satisfactorily. Accordingly, such fibres could not be used in a sanitary pad or in a disposable diaper.
The quantity of water retained in the fibres after hydrolysis can be even further reduced by carrying out the hydrolysis in a non-swelling environment, for example, by carrying out the hydrolysis in an alcohol/water system. After hydrolysis, the liquid retained can be as low as 2-3 g/g fibre. Such fibres are then solvent-exchange dried with alcohol into a fluffy mass. But again, while the fibres will reswell fully in water, they will show very little swelling when placed in 1% NaCl solution. Accordingly, such fibres could not be used in a sanitary pad or in a disposable diaper.
As noted above, for uses such as, for example, diapers, sanitary napkins, etc., absorbency of aqueous 1% NaCl is more important than absorbency of water.